Chemical polymerization of vinyl or acrylate based resins via a free radical polymerization mechanism at ambient temperature is traditionally achieved using a binary, redox curing system consisting of a peroxide and an aromatic tertiary amine. On the other hand, light-activated polymerization proceeds via the generation of free radicals from the activation of a photoinitiator, usually an α-diketone, to its excited triplet state. This is followed by the reduction of the activated photoinitiator by an amine accelerator to form an intermediate excited complex (exciplex), which releases the free radicals on dissociation. There was evidence to suggest that bond strengths of resin composites to dentin were influenced by the compatibility of the polymerization modes between adhesive systems and resin composites (Swift E J, May K N, Wilder A D. Journal of Prosthodontics, 7: 256-60, 1998). A report further revealed that common light-cured, self-etching adhesive systems were incompatible with chemical-cured composites (Miller M B et al. Realty 13(1), 182-7, 1999), to the extent that no effective bonding was achieved for some systems. However, the systems that bonded poorly to the chemically cured composites exhibited high shear bond strengths with the use of light-cured resin composites. Generally, the nature of incompatibility of self-etching adhesives with chemical-cured resins is attributed to the in situ reaction between the basic component (amine coinitiatior) of the dual-cure restorative and the acid components of the adhesive system. More specifically, these acid components of the bonding agent are able to readily protonate the tertiary aromatic amine, which could be found in the self-curing resin composite as part of the organic redox catalyst. The protonated amine (quaternary aromatic amine) became inactive and/or not reactive towards the peroxide. Consequently, initiating radicals were able to be generated under ambient conditions. Overall, such a catalyst pair would have a loss in efficiency, and the rate and degree of functional group conversion are significantly diminished compromising the performance of the dental adhesive. In order to avoid such an unwanted amine protonation reaction, the dental restoratives to be used in combination have to be limited to those of the photo-curable type only.
Although light-cured resin composites have largely superseded the use of chemical-cured composites in esthetic dental applications, chemically activated composites still have important applications in contemporary restorative dentistry. The longer working time of chemically cured composites has been adopted in the ‘directed shrinkage technique’ for posterior resin composite restorations. In this technique, a slow setting, chemically cured composite was used either in bulk or as a basal layer to relieve the stress developed in a restoration by the flow of the partially polymerized material. Chemically cured resins are frequently used as restorative materials in areas that are not easily penetrable by light, and as auto- or dual-curing resin cements for luting of crown and bridges, inlays and onlays along with endodontic posts. In order to facilitate the use of light curing self-etching bonding agents with dual curing or chemically curing composites, a self-curing activator is required to overcome the incompatibility of the acid containing adhesive with the amine in the redox catalyst of the chemically cured system. In the Prime&Bond NT® Dual-Cure bonding system available from DENTSPLY International, Inc., the regular light-cure bonding agent, Prime&Bond NT® is mixed with Self-Cure Activator prior to use. The Prime&Bond NT® Dual-Cure exhibits excellent bond strength when bonding a dual-cure cement, e.g. Calibra® available from DENTSPLY International, Inc., in chemical-cure mode.
The Self-Cure Activator developed specifically for Prime&Bond NT® does not necessarily work as well with other simplified adhesives. More importantly, Self-Cure Activator has to be mixed with a simplified adhesive prior to use, which is an extra step for clinicians. There is a real need to simplify the system. It was thought that cement incorporating a base could neutralize acids in a simplified adhesive to mitigate or eliminate the issue of deactivation of amine in the cement. As a result, the incompatibility of a simplified adhesive and resin cement in self-cure mode may be reduced or eliminated.
Osada et al (Dental materials Journal, 21(11): 1044-50, 2005) reported that addition of an anion exchange resin to the amine component of self-cured resin was an effective means of enhancing the bond strength on dentin and to prevent amine neutralization through the acid groups of self-etching primer adhesives. The drawback with this approach is that flexural modulus of cement with anion exchange resin drops off. More critically, anion exchange resin is polymerized beads with diameter 1-2 mm, which would result in cement with film thickness too large for any clinical usage.